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Hubble has made another discovery about the planets in our Solar System during its routine yearly monitoring — unexpected weather formations that give us clues about the atmospheres of Uranus and Neptune.

As on Earth, other planets in the Solar System also have seasons caused by the axial tilt of the planet and its variable distance from the Sun (due to orbital eccentricity, or the oval-shaped orbit of planets which moves them closer to the Sun at some times and further away at others). Uranus and Neptune have much longer seasons than Earth, with seasons that last for decades rather than months.

This means that Hubble is still gathering data about seasonal changes on these planets. In 2007 the northern hemisphere of Uranus came out of a winter which lasted for 42 years and is followed by a 42 year-long summer. And there’s a storm brewing there — a huge stormy cloud cap which is covering the north pole and a good chunk of the planet’s surface, which can be seen in the image below. In the case of Neptune, a dark splotch can be seen which also indicates a storm, although in this case it is a storm vortex with white companion clouds nearby.

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The white cap visible over the polar region of Uranus (left) and a dark storm over Neptune (right) NASA, ESA, A. Simon (NASA Goddard Space Flight Center), and M.H. Wong and A. Hsu (University of California, Berkeley)

Uranus is believed to be experiencing this massive storm because it has a unique rotation pattern. It has an extreme degree of tilt, shifted almost completely over on its side, so during summer the Sun shines almost straight down onto the north pole and never sets. As the northern hemisphere approaches the middle of its summer season, the polar cap region is directly exposed to the Sun which drives seasonal changes in atmospheric flow.

As for Neptune, this is not the first time that storms have been observed on the planet. The Voyager 2 craft observed two dark storms as it flew past in 1989, and scientists have calculated that dark spots appear on the planet around every four to six years, lasting around two years each. The dark vortex is accompanied by white clouds due to the movement of air as it flows over the storm, causing methane gas to freeze into ice crystals and forming clouds similar to the ones we see here on Earth.

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